1. Field of the Invention
The present invention relates to a semiconductor memory device and a method for producing the same. In particular, the present invention relates to a semiconductor memory device incorporating a capacitor which includes a lower electrode, a dielectric film, and an upper electrode, the capacitor being electrically connected to a selection transistor via an electrically conductive plug and a diffusion barrier film, as well as a method for producing the same. The present invention further relates to a method for producing a capacitor for use in such a semiconductor memory device, the capacitor including a ferroelectric film (hereinafter, such a capacitor will also be referred to as a xe2x80x9cferroelectric capacitorxe2x80x9d), as well as a method for producing a nonvolatile semiconductor memory device incorporating such a ferroelectric capacitor.
2. Description of the Related Art
Ferroelectric materials have a large range of applications for various devices utilizing their functions including spontaneous polarization, high dielectric constant, electro-optical effects, piezoelectric effects, pyroelectric effects, and the like. For example, the pyroelectric properties of ferroelectric materials are utilized in infrared linear array sensor applications; their piezoelectric properties are utilized in ultrasonic wave sensor applications; their electro-optical effects are utilized in waveguide type optical modulator applications; their high dielectric properties are utilized in capacitors for use in DRAMs (Dynamic Random Access Memories) and MMICs (Monolithic Microwave Integrated Circuits), and so on.
Above all, non-volatile memories (FRAMs: ferroelectric non-volatile memories), which incorporate a ferroelectric film and are capable of high-density implementation and high-speed operation, have been a subject of intensive development, in response to the recent development in thin film formation technologies in combination with semiconductor memory art. FRAMs provide advantages such as high-speed writing/reading, low voltage operation, and high endurance through repetitive writing/reading. Therefore, research and development efforts have been made to implement FRAMs which can replace not only conventional non-volatile memories but potentially SRAMs and/or DRAMs also.
Conventional non-volatile memories such as EPROMs, EEPROMs, and flash memories require a read time which is equivalent to that required for a DRAM. However, they require a long write time, thereby hindering high-speed operation. On the other hand, FRAMs are capable of both reading and writing at a rate which is equivalent to that of DRAMs. Thus, high-speed operation is expected of FRAMs. In a typical device structure of an FRAM, one cell is constructed from one selection transistor and one ferroelectric capacitor; alternatively, one cell is constructed from two selection transistors and two ferroelectric capacitors.
Conventionally, oxide ferroelectric materials (PZT (lead zirconate titanate), SrBi2Ta2O9, Bi4Ti3O12, etc.) have been studied as ferroelectric materials for use in a ferroelectric capacitor. As a lower electrode of such a ferroelectric capacitor, electrodes formed of a precious metal material (e.g., Pt, Pt/Ta, Pt/Ti) or composite electrodes formed of precious metal material(s) and a closely contacting film have been used for improved characteristics of the resultant thin ferroelectric film.
A ferroelectric film must be in the form of a crystallized film in order for its functions to be fully utilized. Therefore, a high-temperature heat treatment at about 600xc2x0 C. to about 800xc2x0 C. in an oxygen atmosphere is required as a crystallization process.
Furthermore, it is generally considered essential to employ a stacked structure in order to achieve a high density integration, e.g., 4 Mbits or above, by utilizing such a ferroelectric capacitor and manufacturing processes thereof. This in turn requires a structure in which a selection transistor is electrically coupled to a ferroelectric capacitor via an electrically conductive plug, e.g., polysilicon. In the case of a Pt/polysilicon structure, a diffusion barrier film (e.g., TiN) is required so as to prevent silicidation of a Pt lower electrode which may occur during a crystallization process of the ferroelectric material.
However, although a Pt film itself has sufficient anti-oxidation properties, the TiN layer in a Pt/TiN/Ti structure may be oxidized by oxygen gas which has moved along grain boundaries in the Pt film during the crystallization process of the ferroelectric material, as reported in the Extended Abstracts of the 43rd Spring Meeting (1996) of the Japanese Society of Applied Physics and the related Societies, 28p-V-6 (p. 500). Furthermore, as reported in the above publication as the article 28p-V-7, oxidation of TiN, if it occurs, may cause peeling at the Pt/TiN interface or hillocks in an upward direction within the Pt film so as to alleviate any variation in stress due to volume expansion resulting from the oxidation of TiN. This presents a considerable problem.
The aforementioned movement of oxide through the Pt film presents another problem in that, in the case where a closely contacting film is employed in a Pt/Ta/TiN/Ti structure or a Pt/Ti/TiN/Ti structure, an insulating material may be formed as a result of oxidation of the Ta or Ti immediately underlying the Pt, thereby disrupting the electrical connection. The lowermost Ti film is a requirement in these multi-layer structures to establish contact between the respective structure and polysilicon.
Thus, constructing an electrode only from a Pt film, or a combination of a Pt film and a closely contacting film can result in marked problems associated with hillocks and/or insufficient electrical contact due to the oxidation of a diffusion barrier film such as TiN. These problems make it difficult to realize stacked type structures.
On the other hand, the use of an oxide electrode material (e.g., IrO2, RuO2, RhO2, OSO2, and ReO2) for a lower electrode under the aforementioned oxide ferroelectric film has begun to be studied because they provide excellent barrier properties and excellent matching with an overlying oxide dielectric material.
Among others, the use of IrO2 can greatly improve the fatigue characteristics of a layer of PZT formed upon an Ir/IrO2/polysilicon electrode or a Pt/IrO2/polysilicon electrode, as reported in Appl. Phys. Lett., vol. 65 (1994), pp. 1522-1524 and Jpn. J. Appl. Phys., vol. 33 (1994), pp. 5207-5210, which ascribes such improvement to the barrier properties of the IrO2 film against the elements (e.g., Pb) composing the ferroelectric film. However, again such a structure is susceptible to the problem of insufficient electrical contact due to oxidation of the polysilicon at the IrO2/polysilicon interface, as well as silicidation of IrO2 formed immediately above the polysilicon, during the IrO2 film formation and the ferroelectric film formation.
An IrO2(1000 xc3x85)/Ir(500 xc3x85)/TiN/Ti lower electrode, incorporating a TiN film as a barrier metal for an oxide electrode (IrO2 electrode), has been reported in the Extended Abstracts of the 43rd Spring Meeting (1996) of the Japanese Society of Applied Physics and the related Societies, 28p-V-4 (p. 499) for solving the problem associated with the reaction between Ir or IrO2 and polysilicon. In this reference, contact characteristics between a silicon substrate having a reduced resistance due to ion implantation and a SrTiO3 film serving as a high dielectric film was examined. As a result, it was confirmed that ohmic contact is established therebetween. More specifically, a leakage current density of about 10xe2x88x927 A/cm2 and a dielectric constant of about 216 are reported to be obtained, both of these values being substantially equal to those obtainable for the Pt electrode. Such an IrO2/Ir/TiN/Ti structure prevents any degradation in electric characteristics of the capacitor which is associated with hillocks and reduced flatness typically caused in the relatively low-temperature process of about 200xc2x0 C. to about 450xc2x0 C. to be performed for forming a SrTiO3 film as a high dielectric material film. Thus, the IrO2/Ir/TiN/Ti structure was confirmed to be a promising candidate for use in the stacked type structure incorporating a high dielectric capacitor.
However, the ferroelectric material crystallization process requires the use of an oxygen atmosphere at a temperature of about 600xc2x0 C. or higher even for forming a PZT film. Moreover, an oxygen atmosphere at about 800xc2x0 C. or higher is often used for a SrBi2Ta2O9 film. In the Pt/TiN/Ti structure, an insulating material may be formed at such a high temperature as a result of oxidation of the Ti which is a closely contacting film, thereby disrupting the electrical conduction. In addition, hillocks may be produced by the film stress due to oxidation of TiN. In the IrO2/Ir/TiN/Ti structure as well, hillocks may be produced by the film stress due to the crystallization process at a high temperature (i.e.,  greater than  about 600xc2x0 C.).
In view of the aforementioned problems, Ir and IrO2 having excellent crystallinity are required to obtain improved anti-oxidation properties. However, when the IrO2 film is formed directly on the Ir film at a high temperature, the IrO2 film becomes non-uniform in quality, making it impossible to form a homogenous film at a high temperature. Moreover, hillocks or the like may be produced in a high-temperature atmosphere due to the poor anti-heat properties of the diffusion barrier film.
Thus, in order to practically produce the memories incorporating a ferroelectric film or high dielectric film in the stacked type structure, it is desirable to implement a lower electrode structure exhibiting no reaction with ferroelectric materials to be provided thereon and having anti-heat properties in an oxidation atmosphere at about 600xc2x0 C. or higher; a flat, dense profile; and a resistivity which is equivalent to or better than that of the conventional Pt electrode.
SrBi2Ta2O9 has been actively studied as a promising candidate of a ferroelectric material for use in the ferroelectric capacitor, because it has better fatigue characteristics and requires lower driving voltage as compared to the conventionally used PZT. Unlike the conventional ferroelectric materials such as PZT, SrBi2Ta2O9 is crystallized through heat-treatment in a high-temperature oxidizing atmosphere of about 700xc2x0 C. to about 800xc2x0 C. in any of the formation methods including an MOD (Metal Organic Decomposition) method, a sol-gel method, an MOCVD (Metal Organic Chemical Vapor Deposition) method and a sputtering method, as disclosed in Japanese Laid-Open Publication Nos. 8-23073 and 9-36309.
It should be noted that such heat-treatment in the high-temperature oxidizing atmosphere does not cause a serious problem in the case of a relatively poorly integrated ferroelectric capacitor with a flat structure. However, in the stacked type structure which is essential to achieve a high-density integration of the ferroelectric memories, a polysilicon plug typically used to establish contact with a lower electrode and/or a diffusion barrier layer (barrier metal layer), e.g., TiN or TaSiN, for preventing diffusion between the plug and the lower platinum electrode, tend to be oxidized during the high-temperature process. Such oxidation of the polysilicon plug and/or diffusion barrier layer (barrier metal layer), if it occurs, may disrupt electrical conduction between the plug and the lower electrode, and/or may cause peeling due to expansion of the barrier metal.
A semiconductor memory device of the present invention includes: a capacitor formed on a substrate and including a lower electrode, a dielectric film and an upper electrode; a selection transistor formed at the substrate; an electrically conductive plug for providing electrical connection between the selection transistor and the capacitor; and a diffusion barrier film provided between the electrically conductive plug and the lower electrode of the capacitor. The diffusion barrier film is a TaxSi1xe2x88x92xNy film or a HfxSi1xe2x88x92xNy film (where 0.2 less than x less than 1 and 0 less than y less than 1), and the lower electrode includes an Ir film and an IrO2 film which are sequentially formed.
The lower electrode may further include an electrically conductive film formed on the IrO2 film, the electrically conductive film containing at least one of metal elements selected from the group consisting of Pt, Ir, Ru, Rh, Os and Re.
The diffusion barrier film may be a TaxSi1xe2x88x92xNy film or a HfxSi1xe2x88x92xNy film (where 0.75 less than x less than 0.95 and 0.3 less than y less than 0.5).
A ratio of a thickness d1 of the IrO2 film to a thickness d2 of the Ir film may satisfy a relation of 1 xe2x89xa6d1/d2xe2x89xa63.
Another aspect of the present invention provides a method for producing a semiconductor memory device including a capacitor which includes a lower electrode, a dielectric electrode and an upper electrode. The capacitor is electrically connected to a selection transistor via an electrically conductive plug and a diffusion barrier film. The method includes the steps of: forming the diffusion barrier film at a prescribed position on a substrate; forming an Ir film on the diffusion barrier film; forming an initial film with a thickness of about 50 xc3x85 to about 300 xc3x85 on the Ir film at a temperature of about 300xc2x0 C. to about 400xc2x0 C., the initial film containing at least one of metal elements selected from the group consisting of Ir, Ru, Rh, Os and Re; forming an IrO2 film on the initial film to obtain a layered structure of the lower electrode; forming the dielectric film on the lower electrode; and forming the upper electrode on the dielectric film.
The method may further include the step of forming an electrically conductive film on the IrO2 film. The electrically conductive film contains at least one of metal elements selected from the group consisting of Pt, Ir, Ru, Rh, Os and Re.
The IrO2 film may be formed at a temperature of about 450xc2x0 C. to about 700xc2x0 C.
A ratio of a thickness d1 of the IrO2 film to a thickness d2 of the Ir film may satisfy a relation of 1 less than d1/d2 less than 3.
The initial film may be formed as an IrO2 film.
The diffusion barrier film may be formed as a TaxSi1xe2x88x92xNy film or a HfxSi1xe2x88x92xNy film (where 0.2 less than x less than 1 and 0 less than y less than 1). Preferably, the diffusion barrier film is formed as a TaxSi1xe2x88x92xNy film or a HfxSi1xe2x88x92xNy film (where 0.75 less than x less than 0.95 and 0.3 less than y less than 0.5).
Still another aspect of the present invention provides a method for producing a semiconductor memory device including a capacitor which includes a lower electrode, a dielectric film and an upper electrode, in which the capacitor is electrically connected to a selection transistor via an electrically conductive plug and a diffusion barrier film. The method includes the steps of: forming the diffusion barrier film at a prescribed position on a substrate; forming an Ir film on the diffusion barrier film; forming the dielectric film on the Ir film using a material which contains oxygen; and forming the upper electrode on the dielectric film.
The diffusion barrier film may be formed as a TaxSi1xe2x88x92xNy film or a HfxSi1xe2x88x92xNy film (where 0.2 less than x less than 1 and 0 less than y less than 1). Preferably, the diffusion barrier film is formed as a TaxSi1xe2x88x92xNy film or a HfxSi1xe2x88x92xNy film (where 0.75 less than x less than 0.95 and 0.3 less than y less than 0.5).
In the aforementioned production method of the present invention, the dielectric film may be a ferroelectric film, and the step of forming the ferroelectric film may include the step of crystallizing the ferroelectric film by conducting heat-treatment in an inert gas atmosphere.
In such a case, the method may further include the step of conducting heat-treatment for supplementing oxygen loss of the ferroelectric film, after the step of crystallizing the ferroelectric film, wherein the heat-treatment is conducted in an oxygen atmosphere at such a temperature that prevents an underlying layer of the lower electrode from being oxidized. The heat-treatment for supplementing the oxygen loss may be conducted after the formation of the upper electrode.
The step of forming the ferroelectric film may include the steps of: applying a constituent material of the ferroelectric film to a prescribed thickness by a film application method; drying the applied constituent material; and repeating the applying step and the drying step a prescribed number of times to form the ferroelectric film having a desired thickness.
Alternatively, the step of forming the ferroelectric film may include the steps of: forming a film of a constituent material of the ferroelectric film to a prescribed thickness; crystallizing the film of the constituent material by conducting heat-treatment in an inert gas atmosphere; and repeating the film forming step and the crystallizing step a prescribed number of times to form the ferroelectric film having a desired thickness. The film of the constituent material of the ferroelectric film may be formed by a film application method.
The ferroelectric film may be formed from a Bismuth-layer-structured-family compound. For example, the Bismuth-layer-structured-family compound may be SrBi2(Ta1xe2x88x92xNbx)2O9 (where 0 less than x less than 1).
The heat-treatment for crystallizing the ferroelectric film may be conducted at a temperature of about 650xc2x0 C. to about 800xc2x0 C.
The method may further include the steps of: forming the selection transistor at the substrate; forming an interlayer insulation film on the selection transistor; and forming a contact hole in the interlayer insulation film and forming the electrically conductive plug in the contact hole, wherein the diffusion barrier film is formed on the electrically conductive plug in the semiconductor substrate so as to be electrically connected to the electrically conductive plug.
Thus, the invention described herein makes possible the advantages of (1) providing a semiconductor memory device incorporating a ferroelectric capacitor which exhibits excellent ferroelectric characteristics, and (2) providing a method for producing such a semiconductor memory device with high yield.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.